Unfortunately I tired both methods previously and still got the same error.
As nc50lc has pointed out, if after import of the private key to Bitcoin Core you get an encoding error, it means that the WIF (Wallet Import Format) is invalid.
There are no I, l, O, or 0 in the private key
I'd bet that you've misread a character, or written a character wrongly. If it's a 51-character long private key, that starts with 5, and has no I, l, 0, and O, then there
has to be something wrong with you, or with the software that generated it back then.
Please confirm it for me: do you feel 100% confident that you can't have possibly made a mistake during writing, and it's crystal clear what the characters are?
So what happened is I used the old 'Bitcoin Wallet Recovery' tool I linked in my OP to search for this wallet on an old formatted drive. It managed to find one wallet, but it was corrupted.
Then I used pywallet to dump the keys from the corrupted wallet.dat file that was found. In the wallet.text file that pywallet generated, I found this 51 character key, along with an address. The address is valid and had one transaction as per blockchain explorer all the way back from 2011. Its a miniscule amount, though worth recovering as I am taking this as a learning experience incase I find the real missing wallet.
So not sure where things have gone wrong in the process, potentially the wallet.dat is incomplete or partially overwritten and pywallet generates an incorrect key because of this. For all I know the address could be incorrectly generated as well. But what are the odds of generating a random public address that has some balance
![Shocked](https://bitcointalk.org/Smileys/default/shocked.gif)
. I have no records of this address but it suits the timeline for the wallet I was originally looking for but not the amount.